1. Field of the Invention
This invention relates to the field of integrated optics, and more particularly to methods of monolithic integration of active devices with passive components.
2. Description of Related Art
Presently many optoelectronic systems are assembled from separate components that are individually packaged into fiber modules. These components include, but are not limited to, LEDs, lasers, amplifiers, modulators, detectors, power splitters, switchers, filters, and multiplexers. However, the cost of the components is high mainly because of the package itself, where coupling optics, temperature stabilization, and precise adjustment are all required. Moreover, systems based on the discrete components are power consumable and it is difficult to make them compact in size. Joining the components into a single-package configuration, also known as a photonic integration circuit (PIC), eliminates these disadvantages.
Photonic integration circuits can be based either on hybrid or on monolithic integration. Hybrid photonic integrated circuits bring together optical devices based on different material systems, for example, an III-V evanescent laser bonded on Si (A. W. Fang et al., “Electrically pumped hybrid AlGaInAs-silicon evanescent laser”, Optical Express, vol. 14, 9203-9210, 2006). An advantage of hybrid integration is that each component is optimized for one specific function, enabling deployment of state of the art components. However, there are also disadvantages including, but not limited to, an inefficient light coupling between the components, different lattice and thermal expansion constants, and diffusion of impurities between the components.
On the other hand, monolithic integration joins the devices based on the same material system, avoids aligning and bonding problems, and provides exceptional thermal and mechanical characteristics (see for example U.S. Pat. No. 7,282,311 by Little, issued Oct. 16, 2006). Taking into account these benefits, monolithic integration can be preferable for certain applications with modest integration levels.
Low-loss optical waveguides are normally needed in PICs for interconnection and also for some passive components, e.g. spectral and spatial filters, splitters, delay lines, and chromatic dispersion compensators. There are a few approaches for monolithic integration of the passive waveguides including different regrowth technologies, quantum well intermixing, and vertical twin-waveguide structure growth.
The most straightforward passive waveguide integration technique is epitaxial growth of a second waveguide with the desired properties after the removal of the original waveguide, also known as the butt joint regrowth method (see U.S. Pat. No. 4,820,655 by Noda, issued Apr. 11, 1989). The main advantage of this integration scheme is a high degree of flexibility in the design, for example, compositions, thicknesses, and doping concentrations. However, the epitaxial crystal growth at the abutting locations creates the problem of layer misalignment and imperfect interfaces (quality and shape) between the active and passive components, which results in scattering loss, parasitic optical feedback, and low coupling efficiency. Another regrowth approach, selective area growth, uses a dielectric mask to inhibit epi-layer growth during metal organic vapor phase epitaxy (MOVPE) or metal organic chemical vapor deposition (MOCVD) and, as a result, to tailor the waveguide properties along its length (see U.S. Pat. No. 5,543,353 by Suzuki, issued Aug. 6, 1996 and U.S. Pat. No. 7,060,615 by Glew, issued Jun. 13, 2006). However, the waveguide properties cannot be strongly changed on a short distance resulting in additional absorption losses and chirp in the region of the band edge transition. Moreover, a very precise control of growth parameters is necessary.
Another passive waveguide integration method is based on disordering of quantum wells, also known as quantum well intermixing (QWI), to locally change band-edges (see U.S. Pat. No. 6,989,286 by Hamilton, issued Jan. 24, 2004). Since the QWI process only slightly modifies the composition profile and does not change the average composition, there is a negligible refractive index discontinuity at the interface between adjacent sections. Different modifications of the QWI technique, such as impurity free vacancy disordering (IFVD), impurity induced disordering (IID) and laser-induced disordering (LID), suffer from their specific drawbacks, including free-carrier absorption, parasitic conductivity, residual damage from implantation, inferior quality of recrystallized material after laser melting, and degradation of the top surface caused by high-temperature annealing. Taking into account complexity, irreproducibility, and the poor area selectivity of the intermixing process, QWI technology is not a practical method for monolithic integration of multi-functional optoelectronic devices in PIC (J. H. Marsh, “Quantum well intermixing”, Semiconductor Science Technology, vol. 8, pp. 1136-1155, 1993).
Vertical twin waveguide structure represents a promising integration platform technology. This integration technique can be realized by using either the waveguide modes beating concept or an adiabatic mode transformation concept. In the first case, the power transfer results from the bimodal interference between two supermodes of the vertical twin-waveguide (TG) structure (Y. Suematsu et al., “Integrated twin-guide AlGaAs laser with multiheterostructure”, IEEE Journal of Quantum Electronics, vol. 11, pp. 457-460, 1975; see also U.S. Pat. No. 5,859,866 by Forrest, issued Jan. 12, 1999). Despite the fact that active and passive functions are separated into different vertically displaced waveguides, all integrated components cannot be well optimized separately due to a requirement of resonant coupling of both waveguides. Moreover, performance characteristics of the devices based on the TG structures are not stable due to mode interaction and fluctuation in the device structure itself (layer thickness, composition, dry etching profiles). On the contrary, the adiabatic mode transformation concept, based on an asymmetric twin-waveguide (ATG) with tapered couplers, is unaffected by modal interference (see U.S. Pat. No. 6,282,345 by Schimpe, issued Aug. 28, 2001). The waveguide is designed in such a way that only one mode exists. To reduce coupling losses during the power transfer process, the lateral tapering of the active waveguide at a junction of the active-passive waveguides is used (see U.S. Pat. No. 5,078,516 by Kapon, issued Jan. 7, 1992). As the active waveguide rib is narrowed, the mode profile is smoothly transformed without any loss of power and, finally, the mode is adiabatically pushed down into the passive waveguide. This allows the independent optimization of the active/passive devices in a single epitaxial growth step. However, there are strict requirements for the etching process (at least two steps), and for the precision of sub-micron lithography with a complicated alignment procedure. In addition, ridge waveguides are rather long, and precise control of taper tips is required.
Each of the above-mentioned coupling techniques suffers from one or more of the following major drawbacks: high optical/coupling losses, poor manufacturability, high cost, insufficient reproducibility, and inadequate reliability. Therefore, there is a need in the art for a novel economical and manufacturable active-passive coupling technique that permits further progress in photonic-network communication technology.